Various types of sheet conveying rollers are incorporated in sheet conveying mechanisms provided, for example, in electrostatic copying machines, laser printers, plain paper facsimile machines, copier-printer-facsimile multifunction machines, inkjet printers, automatic teller machines (ATM) and the like.
Examples of the sheet conveying rollers include sheet feed rollers, transport rollers, platen rollers and sheet output rollers, which are each adapted to be rotated in frictional contact with a sheet to convey the sheet. The term “sheet” is herein defined to include a paper sheet, a plastic film sheet and a sheet made of a material other than paper, and this definition is effective in the following description.
Typical examples of a material conventionally used for such a sheet conveying roller include various rubbers such as a natural rubber (NR), a urethane rubber, an ethylene-propylene-diene rubber (EPDM), a polynorbornene rubber, a silicone rubber and a chlorinated polyethylene rubber.
However, paper dust generated from paper sheets is liable to adhere to an outer peripheral surface of the sheet conveying roller. As the sheet conveying roller is repeatedly brought into contact with the paper sheets, the paper dust is accumulated on the outer peripheral surface of the sheet conveying roller. This reduces the friction coefficient of the sheet conveying roller with respect to the paper sheets, thereby causing sheet conveying failure at a relatively early stage.
Particularly, paper sheets having a higher ash content is more liable to generate paper dust, so that the accumulation of the paper dust and the sheet conveying failure attributable to the accumulation of the paper dust are more liable to occur.
It is contemplated that a rubber mixture containing an EPDM, an isoprene rubber (IR) and a styrene butadiene rubber (SBR) in a predetermined ratio is used as a rubber component for formation of the sheet conveying roller (see JP-2011-116480A).
Based on the contemplation, the inventor of the present invention finds that a sheet conveying roller produced by using the three rubbers in combination as the rubber component suppresses the accumulation of the paper dust and the accompanying sheet conveying failure to some extent and ensures proper sheet conveyance for a longer period of time than ever.
However, the effects of the combinational use of the three rubbers as the rubber component are not sufficient. There is still a demand for a sheet conveying roller which prevents the accumulation of the paper dust and the accompanying sheet conveying failure for a longer period of time to ensure proper sheet conveyance.
In the combinational use of the three rubbers as the rubber component, the IR and the SBR are added to the EPDM intrinsically having a smaller friction coefficient in order to improve the initial friction coefficient of the sheet conveying roller. However, the effect of improving the initial friction coefficient is not sufficient.
Of the three rubbers as the rubber component, the EPDM is intrinsically excellent in weather resistance as well known. However, the IR and the SBR each have a carbon-carbon double bond in a main chain thereof and, therefore, are insufficient in weather resistance as compared with the EPDM. This may result in cracking and breakage of the sheet conveying roller during prolonged use of the sheet conveying roller.
Further, the rubber composition has a complicated formulation, because a lot of ingredients including the three rubbers as the rubber component should be blended. This reduces the productivity of the rubber composition and the productivity of the sheet conveying roller to be produced by using the rubber composition, and makes it difficult to control the qualities of the rubber composition and the sheet conveying roller. Further, this increases the production costs.